The present invention relates to a process for preparing grit blasting particles coated with titanium dioxide by sinter-fusing titanium dioxide on the surface of grit blasting particles made of a base material.
Grit blasting particles coated with titanium dioxide are described in DE 298 21 398 and in Biomedizinische Technik, vol. 49, supplement 1, 35 (1995), and also a general process for preparing them. According to this, these types of titanium dioxide-coated particles are prepared by sinter-fusing an aqueous TiO2 slip suspension on the material making up the particles. The particles are first intimately mixed with the TiO2 slip suspension, the solvent is removed and then the slip which has dried onto the particles is sinter-fused at temperatures between 300 and 1400xc2x0 C. These types of titanium dioxide-coated particles, preferably those made of inert corundum (aluminum oxide, Al2O3) as base material, are mainly used to grit blast, and simultaneously tribochemically coat with titanium dioxide, components for use in the human body and in particular in the oral cavity. When preparing components made from metallic materials for this type of use, such as in particular in dentistry when manufacturing frameworks made of dental alloys for dental repairs, grit blasting is a conventional working step for preparing the surface. Corundum is generally used as the grit blasting material in this case.
When manufacturing dentures, prosthetic metal frameworks such as crowns or bridges are coated with a tooth-coloured facing material in order to produce a more pleasing appearance. In particular in the case of removable dentures and long-term temporary products, plastic materials are preferably used as the facing material, rather than ceramics, due to their typical materials properties such as, for example, higher elasticity. Since the properties of the facing materials based on plastics clearly differ from those of the alloys from which the metal frameworks are manufactured, a permanent bond cannot be ensured without the use of adhesive bonding systems.
The extreme conditions in the oral medium cause problems, wherein the materials are subjected to moisture, temperature changes and mechanical strain as well as the individual""s personal habits such as mode of eating and the taking of any medicaments. As a result of these effects, a peripheral gap often forms after a short time between the metal framework and the plastic facing. Discoloration occurs in this peripheral region due to the onset of metal corrosion and penetration by microorganisms. The different coefficients of thermal expansion of metal and plastics and the polymerization shrinkage of the latter, can produce certain stresses which, assisted by the formation of peripheral gaps and the effects of moisture and also mechanical strain, can lead to damage or even to loosening of the facing.
In principle, known processes for producing strong and as gap-free as possible composites of metal and plastics are substantially based on applying, in a first step, a siliceous layer to the metal surface which is then silanized in a second step by treatment with a silane bonding agent, for example a functional alkoxysilane. The silane acts as a chemical bonding agent between the silicatized metal surface and the polymeric facing material. The known processes differ substantially in the type and manner of application of the silicate layer. In DE 38 02 043 this is produced by tribochemical coating when grit blasting the metal surface using silicon dioxide-coated corundum as the blasting grit. A process of this type is used in dentistry under the name xe2x80x9cRocatecxe2x80x9d.
From the literature references mentioned at the beginning, it can be seen that tribochemical coating with titanium dioxide has better biocompatibility than siliceous coatings and in combination with a silane bonding agent, due to higher resistance to hydrolysis, leads to a much better adhesive bond between metal and plastic facing.
However, in practice it has been shown that the preparation of this type of titanium dioxide-coated grit blasting particles by sinter-fusing an aqueous TiO2 slip suspension on the particle material does not lead to a sufficiently uniform product. This contains, in particular, particles with varying amounts, thickness and distribution of the TiO2 coating and also free TiO2 particles with a high proportion of fines. This has a disadvantageous effect on anchoring the TiO2 layer in and on the metal surface during the working process of grit blasting and tribochemical coating, wherein in particular the virtually unavoidable proportion of very fine TiO2 particles impairs the resulting metal/plastic adhesive bond.
An object of the present invention therefore is to find and develop a process for preparing grit blasting particles coated with titanium dioxide which does not have the disadvantages described above.
Surprisingly, it has now been found that, in particular for long-term use, substantially better metal/plastic adhesive bond values can be produced if the grit blasting particles used for blasting and at the same time tribochemically coating with titanium dioxide metallic components have been obtained by sinter-fusing titanium dioxide on the grit blasting particles. According to the invention, the coating procedure is performed in such a way that a mixture of finely divided titanium dioxide and grit blasting particles are continuously passed through the flame in a flame reactor, with the aid of conveying air, and the titanium dioxide is sinter-fused on the surface of the grit blasting particles over an average residence time of 0.1 to 5 seconds in a temperature range from 600 to 1200xc2x0 C.
Thus, the invention provides a process for preparing grit blasting particles coated with titanium dioxide by sinter-fusing titanium dioxide on the surface of the grit blasting particles made of a base material which is characterized in that a mixture of finely divided titanium dioxide and grit blasting particles is passed continuously through the flame of a flame reactor, with the aid of conveying air, wherein the titanium dioxide is sinter-fused on the surface of the grit blasting particles over a residence time of 0.1 to 5 seconds in a temperature range from 600 to 1200xc2x0 C.
The core feature of the process according to the invention is that a dry powder mixture is reacted in a flame reactor, wherein the titanium dioxide is sinter-fused on the grit blasting particles by the direct effect of heat in a flame.